154 CORAL FORMATIONS. 
IX. It is an inquiry of some interest, whether, in an archipelago 
like the Paumotus, coral debris is not carried from the coral islands, 
and distributed over the bottom of the ocean; and whether limestones 
thus originating, are not in process of formation. I venture no posi- 
tive assertion on this subject, yet would express strong doubts. ‘The 
fact that soundings off some islands, as we recede from the reef-grow- 
ing depths, lose more and more in the proportion of coral sand, till we 
finally reach a bottom of earth, like the material of the island, bears 
against any such hypothesis. ‘This was found to be the case off 
Upolu, where the reefs are extensive. 
X. It remains still to speak of the proofs of elevation or subsidence 
presented by coral islands throughout the Pacific, and of the former 
extent of Pacific lands compared with the present. But these topics 
relating to the dynamics of the ocean, form a separate chapter, follow- 
ing our geological descriptions of the several groups of islands in the 
Pacific. 
We might dwell also on the formation of caverns by the rains 
becoming subterranean waters; on the illustrations of the action of 
marine currents afforded by this subject; on the agency of polyps in 
rock-making. But the deductions are too obvious to require farther 
remark. 
decomposition, the magnesia taking the place of part of the lime, and the excluded lime 
combining with the sulphuric acid set free. The result is magnesian carbonate of lime, 
(dolomite,) and hydrous sulphate of lime, (gypsum,) the latter being separated, and either 
continuing in solution or solidifying, according to the amount formed or the proportion 
in the water. ‘This author figures specimens from different localities in which gypsum 
and dolomite are intimately associated ; and among them are some of fossil corals. 
The hypothesis of Von Buch is thus set aside. We have a satisfactory explanation of 
the existence of compact magnesian limestones, like those of our Western States, as well 
as of the crystalline dolomites. The latter may have received some additional magnesia 
during the submarine heating that crystallized them. 
The circumstance of a chemical change going on between the carbonate of lime and 
magnesian salt is especially favourable for consolidation. When the coral is a fine 
mud, and the grains are therefore extremely fine, the dolomization might extend to the 
grains themselves, as well as the infiltrating material acting as a cement. But when the 
grains of coral are large, or there are pebbles, the infiltrated material that might be 
magnesian would constitute but a small part of the whole bed. Hence it is obvious that 
such formations in cold waters would seldom in the mass have the proportions of a true 
dolomite, (54°2 of carbonate of lime, to 45°8 of carbonate of magnesia ;) and they would 
attain such proportions under an ocean only during that action of heat required alike for 
crystallization and chemical combination, 
